Old bit of dry grass cut by writer before noon (7)

'old bit' is the definition.
(a historical coin or 'bit')

'dry grass cut by writer before noon' is the wordplay.
'dry grass' becomes 'hay'.
'cut by' indicates putting letters inside.
'writer' becomes 'pen' (something which writes).
'before' is a charade indicator (letters next to each other).
'noon' becomes 'n'.
'pen'+'n'='penn'
'hay' placed around 'penn' is 'HAPENNY'.

'of' is the link.